Currently, there is a tremendous need for integrated and supervised emergency evacuation and fire safety systems. With improvements in technology, oversight agencies have modified the code of standards for implementing and maintaining emergency evacuation and fire safety systems to be more demanding.
For example, the Underwriters Laboratories (UL) has implemented requirements through the standards code UL 1971 that necessitate visual notification appliances, i.e., signaling devices, in the emergency evacuation and fire safety systems to be in sync (i.e., the time difference not exceeding 10 milliseconds between the slowest and the fastest signaling visual devices.)
It is recognized that signal delay occurs as the signal transitions through wiring from one device to another device. Therefore clocks are now common in each visual notification appliance.
Generally, clock sources are subject to two types of clock frequency instabilities: a change in frequency over time (aging drift); and/or a non-linear change in frequency with ambient temperature known (temperature drift). On top of these instabilities, an inherent frequency tolerance exists which affects clock precision.
When each notification appliance has its own clock that is used to pace events, these instabilities may cause notification appliance clocks to lose synchronization with one another; thus notification appliances will not be synchronized, and those viewable from a common point will appear to be flashing randomly with respect to each other.
For example, a typical clock's precision may change as much as 0.7% due to a frequency tolerance of about 0.1%, frequency stability over temperature of about 0.3%, and frequency stability over aging of about 0.3%. This change in precision may result in significant time differences as to when the notification appliances flash.
Conventional safety systems attempted to achieve the required synchronization of the time difference between notification appliances by sending out periodic synchronization signals and ensuring that all notification appliances that are viewable from a common point are on the same signaling line circuit (SLC).